The present invention relates to a method in a cellular radio communications system of minimising the set-up delay for a connection between the stationary part of the system and a particular mobile station roaming in the system, when delivering a mobile terminated message.
In the patent document WO-A1-96/22666 is disclosed a control data transfer device with load reduction for a mobile communications system, which has one digital and/or analogue control channel dedicated to multi-user signals. The document also teaches an efficient method of paging and finding a mobile station. Various kinds of alerting signals are sent out over a wide location area. From the page response signal is then determined whether or not the mobile station is equipped with a multi-user terminal, in order to reduce the amount of later control signalling within the communications system.
Through EP-A2-0,687,118 is previously known a method of global paging with reverse call set-up in a wireless personal communications system. The paging signal is comprised of a caller identification code and a callee identification code. The former code is used for setting up a reverse call from the called mobile station to the call initiator. The originating communications device is, however, billed for the call.
The present invention provides a solution to the problems that are associated with long set-up delays in a cellular radio communications system. It is of particular importance to reduce this initial delay when serving bursty traffic, such as data communication, where short messages are transmitted at irregular intervals.
The solution, which is described in WO-A1-96/22666 relates to a process of locating a mobile station and efficiently setting up a connection to it by studying the page response signal returned from the mobile station. There is not taught or even suggested that any additional data should be included in the page signal itself.
According to EP-A2-0,687,118 a certain type of data, the caller identity, is included in the page signal. However, the sole purpose of this data is to make possible a reverse call from the callee. It is true that the page signal comprises data, but the teachings of the document do not advise including any other kind of data, such as payload information or channel allocation instructions, in the page signal.
One object of the present invention is to further reduce the set-up delay for a message directed to a mobile station, a so called mobile terminated call.
Another object of the invention is to reduce the total transmission time for a mobile terminated message.
Yet another object of the invention is to minimise the radio and network resources requirements for transmitting payload information from the stationary part of a cellular radio communications system to a particular mobile station, once a connection has been established with the mobile station.
According to a first embodiment of the inventive method a message, containing payload information, is to be transferred to a specific mobile station from the stationary part of a cellular radio communications system. The message is divided into one or more data units of a predetermined size. A first data unit of the message is multicast from a central node in the stationary part of the communications system, to at least two base stations in the vicinity of the mobile station. The multicast connection between the central node and the base stations may be either semi-permanent or determined on demand from the central node.
Which particular base stations that are located in the vicinity of the mobile station may be determined by considering which base stations that serve the latest registered location or routing area for the mobile station. However, the vicinity question may also be solved by taking into consideration more precise information regarding the location of the mobile station. A mobile station""s position can, for instance, be calculated from GPS-signals (Global Positioning System) received by the mobile station or by triangulation measure-ments, either on radio signals from at least three nearby base stations received by the mobile station or on radio signals from the mobile station received by three or more base stations.
An initial signal, including the first data unit of the message, is then transmitted to the mobile station from the base stations in the vicinity of the mobile station. After having received the initial signal, the mobile station answers the initial signal and thereby acknowledges correct reception of the first data unit. An unanswered initial signal is interpreted as a negative acknowledgement and induces the communications system to repeat the initial signal.
The method according to the invention is hereby characterised by the features set forth in the characterising clause of claim 1.
According to one advantageous and preferred embodiment of the invention, the initial signal is a page signal, which is sent out from all base stations being responsible for communication with mobile stations within a current location area of the mobile station, that is the intended recipient of the message.
According to another preferred embodiment of the invention, the initial signal moreover includes a notification to the mobile station, of a transmission resource, typically a dedicated channel, which is to be used for transmission of the remaining data units in the message.
According to an alternative embodiment of the invention, a positive acknowledgement, indicating correct reception the first data unit, is returned from the mobile station before a transmission resource is allocated to enable transmission of any further data units in the message.
The fact that, according to inventive method, at least one payload containing data unit is included in the very first signal sent to the mobile station, guarantees a very short set-up delay for a connection between the stationary part of the system and the mobile station, even if the mobile station""s position is not specifically known.
In the case of very short messages being transmitted, one-data-unit messages in particular, the inventive method also results in shortest possible total transmission times, and thus a very efficient usage of the available network and radio resources.